This invention pertains generally to phased array antennas, and particularly to space fed phased array antennas used in radar systems.
It is known in the art that a space fed phased array antenna may be used to advantage in ground-to-air defense systems, such as the system called "PATRIOT," (a registered trademark of the Government of the United States of America). Thus, in the PATRIOT system, a control radar utilizing a first space fed phased array antenna is arranged to illuminate a target (say an attacking aircraft) and to receive echo signals directly reflected from the target and a second antenna is arranged to receive signals retransmitted from a guided missile (referred to hereinafter as the "missile") in flight to intercept the target, such retransmitted signals being analogous to echo signals at the missile. Both the echo signals and the retransmitted signals then are processed to derive guidance command signals that are passed through the second phased array antenna to the missile, ultimately to cause the course of the missile to be adjusted as required to ensure interception of the target. Because of the high levels of radio frequency energy passing through both the first and the second space fed array antennas in the PATRIOT system, controllable ferrite phase shifters are used to determine the phase distribution across the radiating elements making up each one of such antennas. The use of controllable ferrite phase shifters (which are nonreciprocal devices) requires that the control signals for each ferrite phase shifter be changed when the radar is transmitting or receiving radio frequency energy. Further, the noise figure of the radar is degraded by the insertion loss of each one of the ferrite phase shifters. Such loss is particularly important when echo signals are being received.